PM0056 datasheet(70/156 Pages) STMICROELECTRONICS

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The Cortex-M3 instruction set

PM0056

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DocID15491 Rev 5

3.4.8

LDREX and STREX

Load and store register exclusive.

Syntax

LDREX{cond} Rt, [Rn {, #offset}]

STREX{cond} Rd, Rt, [Rn {, #offset}]

LDREXB{cond} Rt, [Rn]

STREXB{cond} Rd, Rt, [Rn]

LDREXH{cond} Rt, [Rn]

STREXH{cond} Rd, Rt, [Rn]

where:

•

‘cond’ is an optional condition code (see Conditional execution on page 57)

•

‘Rd’ is the destination register for the returned status

•

‘Rt’ is the register to load or store

•

‘Rn’ is the register on which the memory address is based

•

‘offset’ is an optional offset applied to the value in Rn. If offset is omitted, the address is

the value in Rn.

Operation

LDREX, LDREXB, and LDREXH load a word, byte, and halfword respectively from a

memory address.

STREX, STREXB, and STREXH attempt to store a word, byte, and halfword respectively to

a memory address. The address used in any store-exclusive instruction must be the same

as the address in the most recently executed load-exclusive instruction. The value stored by

the Store-exclusive instruction must also have the same data size as the value loaded by

the preceding load-exclusive instruction. This means software must always use a

load-exclusive instruction and a matching store-exclusive instruction to perform a

synchronization operation, see Synchronization primitives on page 31.

If a store-exclusive instruction performs the store, it writes 0 to its destination register. If it

does not perform the store, it writes 1 to its destination register. If the store-exclusive

instruction writes 0 to the destination register, it is guaranteed that no other process in the

system has accessed the memory location between the load-exclusive and store-exclusive

instructions.

For reasons of performance, keep the number of instructions between corresponding

load-exclusive and store-exclusive instruction to a minimum.

Note:

The result of executing a store-exclusive instruction to an address that is different from that

used in the preceding load-exclusive instruction is unpredictable.

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